Humans Blamed for Extinction of Mammoths, Mastodons & Giant Sloths

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The latest volley in a long-running debate over why woolly
mammoths, giant sloths, mastodons and cave lions died out
worldwide suggests that humans are to blame.

A new global look at the extinctions of large mammals over the
past 130,000 years finds that the loss of species correlates more
closely with the arrival of humans than with changes in climate,
which some studies have cited as a possible culprit.

"The evidence really strongly suggests that people were the
defining factor," said study leader Chris Sandom, co-founder of
the consulting firm Wild Business Ltd., who completed the work as
a postdoctoral researcher at Aarhus University in Denmark.

A mysterious extinction

Scientists have proposed multiple explanations for why mammoths
and giant sloths no longer roam the planet, as they once did.
(They once inhabited every continent but Antarctica.) Did climate
change drive these animals to extinction? Did an impact
by an asteroid or comet kill them off, as with the dinosaurs?
Could disease have spread like wildfire through the population?
Or were human hunters to blame?

Sandom and his colleagues focused on the two possibilities they
gauged as most likely: climate and humans. (The notion that there
was
an extinction-causing asteroid impact during this time is
very controversial, and direct evidence for a pandemic is
lacking.)

Many archaeologists and paleontologists argue that human hunting
makes little sense as a cause for the extinction, because few
"kill sites," where large fauna were butchered, exist in the
archaeological record. Other researchers argue that there are
plenty of kill sites to suggest that ancient humans were hunting
large mammals in significant numbers. For example, in a 2008
study published in the journal Quaternary International,
scientists argue the 14 definite mammoth and mastodon kill sites
found dating back to the North American hunter-gatherer Clovis
culture around 13,000 years ago do, in fact, represent a large
record when compared with other places where large-animal hunting
is known to have occurred.

Humans versus climate

Sandom and his team gathered records on individual species known
to have gone extinct between 132,000 years ago (at the beginning
of the last interglacial period) and 1,000 years ago. They
focused their analysis not on the continent level, as many
studies have, but country-by-country or even state-by-state, in
large nations like the United States. All told, the researchers
analyzed 177 extinct mammals that had weighed more than 22 lbs.
(10 kilograms).

The researchers then compared the timing of the extinctions with
changes in climate and precipitation, and with human migration.

This fits the human-hunting hypothesis well, he said. Large
animals in sub-Saharan Africa would have had millions of years to
co-evolve with humans as they learned to use tools. When early
humans moved into Europe and Asia with their primitive hunting
methods and weapons, they would have had access to a new
population of animals unaccustomed to their clever ways.

In Australia and the Americas, where humans arrived comparatively
late, the extinctions were the most extreme, Sandom said.

"You've got this very advanced hunter arriving in the system," he
said, not unlike the invasive species that cause native
extinctions today. The researchers did not find a strong overall
relationship between extinctions and climate, except in Eurasia,
Sandom said. Climate there might have interacted with human
arrival in a complicated way, with temperatures determining where
people migrated, he added.

Overall, humans' arrival was responsible for 64 percent of the
variation in extinction rates around the globe, while temperature
changes explained 20 percent of the variation, mostly in Eurasia.

Climate change can stress animals, Sandom said, but climate
variations do not always spell doom for species — animals may
simply alter or restrict their range in order to find a habitat
that sustains them. Humanity may have disrupted this adaptive
process for large mammals, he said.

"That was the final straw," Sandom said. "They couldn't handle
the new predator turning up."

The findings were published today (June 3) in the journal
Proceedings of the Royal Society B. Other research on single
species also has turned up tantalizing clues about the possible
cause of the extinction. For example, a 2012 study suggested that
mammoths died out for a number of reasons, including both
climate change and human hunting, and a study published this year
in the journal Evolution found that the
last few mammoth survivors were under great stress, perhaps
from disease or inbreeding. Moreover, starvation was not
what killed off the saber-toothed cats, according to a 2012
study detailed in the journal PLOS ONE.

Editor's note: Due to an editing error, this
story originally implied that the asteroid that killed the
dinosaurs is the same one theorized to have caused the Late
Quaternary extinctions. This story was updated at 4:22 p.m. on
June 4 to correct the phrasing.